Boss gets totally distracted trying to fight and beat the minions and gets spanked by the towers. And boss fight can range from about 20 sec to about 3 mins. Boss is randomly bugged and will sometimes just die for no reason.
1st Wave: Build everything that is not adept towers or minions, then switch to adept and build in combat.
All waves through 14: keep adept builder in and drop mana from your TB monk to auto-upgrade
Final/Boss Wave: Swap in a minion building summoner, throw down those guys just inside the auras, then swap to another summoner with monk/huntress pets at the main tower stack, or keep adept builder in.
Test run:
Build time 5:00, Combat time 6:00, total 11:00
2nd run: 4:29 build, 5:47 combat = 10:16 total
Best total time so far = 8:34 (3:14 build, 5:20 combat)